Modulasi Sudut (2) Levy Olivia MT
3.3.3 Implementation of Angle Modulators and Demodulators • Design an oscillator whose frequency changes with the input voltage. • Voltage-controlled oscillator • Varactor diode - capacitance changed with the applied voltage. • A inductor with the varactor diode is used in the oscillator circuit.
Let the capacitance of the varactor diode is given by • When m(t) = 0, the frequency of the tuned circuit is given by • In general for nonzero m(t), we have • Assuming that • We have
Indirect method for generation of FM and PM signals • generate a narrow band angle-modulated signal • change the narrow band signal to wideband signal
Generate wideband angle-modulated signals from narrow band angle-modulated signals • frequency multiplier • implemented by nonlinear device and bandpass filters • Using down converter
A nonlinear device followed by a bandpass filter tuned to the desired center frequency can be used as frequency multiplier. • For example, assume a nonlinear device has the function • The output signal will be • The frequency is multiplied by a factor of 2.
FM demodulation • generate an AM signal • use AM demodulator to recover the message signal • Pass the FM signal through a filter with response • If the input to the system is • the output • The above signal is an AM signal.
FM to AM converter: Tuned circuit implementation But, usually the linear region of the frequency characteristic may not be wide enough.
Balanced discriminator • use two tuned circuits • connect in series to form a linear frequency response region.
FM demodulator with phase-locked loop (PLL) • Input : • VCO output: • Phase Comparator:
By taking the Fourier transform • Suppose that we design G(f) such that v(t) is the demodulated signal